Japanese Patent Application Laid-Open No. 2004-211847 discloses a conventional power transmission apparatus applied to, for example, a conveyer or a garbage disposal. The power transmission apparatus can properly control an increase in its transmission capacity and reduction of its noise or vibration in accordance with a load exerted by a driven device, while reducing its size and saving space. The power transmission apparatus can transfer an input power to the driven device, and its power transmission mechanism is composed of a first inscribed meshing planetary gear mechanism and a second inscribed meshing planetary gear mechanism disposed in parallel on a power transmission path between an input shaft and an output shaft. The first inscribed meshing planetary gear mechanism includes a first external gear and a first internal gear which slightly differ in the number of teeth, the first external gear being disposed inside the first internal gear such that the first external gear can rotate eccentrically while maintaining the inscribed meshing. The second inscribed meshing planetary gear mechanism includes a second external gear and a second internal gear which slightly differ in the number of teeth, the second external gear being disposed inside the second internal gear such that the second external gear can rotate eccentrically while maintaining the inscribed meshing. The difference in the number of teeth between the first external gear and the first internal gear is rendered different from the difference in the number of teeth between the second external gear and the second internal gear, whereby the power transmitting characteristic of the first inscribed meshing gear mechanism is rendered different from that of the second inscribed meshing planetary gear mechanism.
A cycloid reduction gear built in an in-wheel motor drive device is disclosed in, for example, the Japanese Patent Application Laid-Open No. 2008-309264. The disclosed cycloid reduction gear includes a casing, an input shaft having an eccentric portion, a revolving member which has a through hole into which the eccentric portion is inserted and which revolves about its rotation axis as a result of rotation of the input shaft, an outer periphery engagement member which is held by the casing and which engages with an outer peripheral portion of the revolving member so as to cause rotation of the revolving member about its axis, and a motion conversion mechanism which includes internal pins provided on an output shaft, holes formed in the revolving member, having a diameter larger than the outer diameter of the internal pins by a predetermined amount, and receiving the internal pins, and cylindrical members fitted onto the internal pins at a position where the cylindrical members come into contact with the inner wall surfaces of the holes. The motion conversion mechanism converts rotational motion of the revolving member about its axis to rotational motion around the rotation axis of the input shaft and transmits the rotational motion to the output shaft. Each cylindrical member is fitted onto an internal pin corresponding thereto with a predetermined radial gap formed therebetween. Dynamic pressure grooves are formed in either of the outer surface of the internal pin and the inner surface of the cylindrical member, whereby a dynamic pressure bearing is constituted.
Incidentally, in the conventional power transmission apparatus, the power transmitting characteristics of the first and second gear mechanisms provided in parallel are rendered different from each other by providing a difference between the difference in the number of teeth between the external gear and the internal gear of the first gear mechanism and the difference in the number of teeth between the external gear and the internal gear of the second gear mechanism. Therefore, the conventional power transmission apparatus is large in size, and rendering the conventional power transmission apparatus light and compact has been difficult.